Mango plants need regular watering, Make sure you're watering your mango plant appropriately, allowing the soil to dry out slightly between watering.
Mango plants require plenty of sunlight to thrive, make sure that your plant is placed in a location where it can receive bright, indirect sunlight for each day.
Yellowing leaves can signal a nitrogen deficiency, while weak stems or slow growth might point to a lack of phosphorus. Consider fertilising your plant with a balanced fertiliser specifically formulated for fruit trees.
Transplant Shock leading to leaf drop, give the plant some time to acclimatise to its new environment.
I observe webs on your plant and this makes me think the problem is spider mites. Spider mites and aphids are the pests causing leaves to droop.
Insecticidal soap and neem oil are your go-to for pest control.
If you're not sure about the causes of the leaf drop or how to cure it, consulting with a gardening expert or a local nursery may be helpful.